Generally, this kind of speed control device applies a rotational resistance to the control lever for overcoming a return spring to thereby stop the chain guide at a predetermined position to maintain a desired speed-change stage.
In this case, movement of a switched chain cannot be sensed by a cyclist. Hence, a positioning device has been proposed which is provided at one member of the linkage mechanism with a positioning plate including a plurality of recesses corresponding to the number of speed change stages and carries a retainer engageable with each recess at another member movable relative to the one member so that the retainer during the speed changing engages with one recess to determine the speed change stage and the cyclist can read out the movement of the chain through the clicking feel from engagement of the retainer with the recess.
In a case where the control wire is operated for changing the bicycle speed, especially when the chain is switched from a larger diameter sprocket to a smaller diameter one, the cyclist, especially an unskilled one, although a positioning mechanism is provided, may stop the speed changing operation before or beyond the latter setting position without accurately meeting the desired position. Hence, the retainer at the positioning mechanism may come to a halt near the border between the recesses, causing the chain to contact with the sprocket on the way of chain switching or to one-sidedly engage with the same, thus creating the problem of generation of noise, or unexpected wear in the sprocket and chain.
Accordingly, it is desirable that the chain, even when the control lever is improperly operated, is automatically exactly switched to the desired sprocket without being kept in improper contact therewith.